Process for producing fluorelastomers

ABSTRACT

Fluoroelastomers having copolymerized units of vinylidene fluoride major monomer, at least one other fluorinated major monomer, and at least one cure site monomer are prepared in an aqueous suspension polymerization process using an initiator consisting essentially of a solution of an oil soluble peroxide in a water-soluble hydrocarbon solvent.

FIELD OF THE INVENTION

This invention pertains to a novel process for the production of afluoroelastomer; more particularly, it pertains to a suspensionpolymerization process for the production of a fluoroelastomercomprising copolymerized units of vinylidene fluoride, units of at leastone other fluorinated major monomer and units of at least one cure sitemonomer and wherein said fluoroelastomer has substantially no ionic endgroups.

BACKGROUND OF THE INVENTION

Fluoroelastomers having excellent heat resistance, oil resistance, andchemical resistance have been used widely for sealing materials,containers and hoses. Examples of fluoroelastomers include copolymerscomprising units of vinylidene fluoride (VF₂) and units of at least oneother copolymerizable fluorine-containing major monomer such ashexafluoropropylene (HFP), tetrafluoroethylene (TFE),chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and aperfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE includeperfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) andperfluoro(propyl vinyl ether).

In order to develop the physical properties necessary for some end useapplications, fluoroelastomers must be crosslinked. Typical curativesfor promoting crosslinking include polyamines, polyols and thecombination of an organic peroxide and a multifunctional unsaturatedcoagent. All these compounds form crosslinks by reacting with a curesite on the fluoroelastomer polymer chain. Examples of cure sitesinclude a double bond, or a labile hydrogen, bromine, iodine, orchlorine atom. A common method of introducing a cure site into afluoroelastomer made by continuous emulsion polymerization is tocontinuously add a minor amount of a copolymerizable cure site monomer,along with the major monomers (e.g. VF₂, HFP, TFE, PAVE, etc.) to thepolymerization reactor. In this manner, cure sites are randomlydistributed along the resulting fluoroelastomer polymer chain. Suitablecure site monomers include bromine- or iodine-containing olefins, andbromine- or iodine-containing unsaturated ethers, non-conjugated dienesand 2-hydropentafluoropropylene (2-HPFP). Alternatively, or in additionto cure site monomers, cure sites may be introduced into thefluoroelastomer by conducting the polymerization in the presence of achain transfer agent containing iodine, bromine or both. In this manner,a bromine or iodine atom is attached to the resulting fluoroelastomerpolymer chain at one or both ends. Such chain transfer agents typicallyhave the formula RI_(n), RBr_(n) or RBrI, where R may be a C₁-C₃hydrocarbon, a C₁-C₆ fluorohydrocarbon or chlorofluorohydrocarbon, or aC₂-C₈ perfluorocarbon, and n is 1 or 2.

Production of such fluoroelastomers by emulsion and solutionpolymerization methods is well known in the art; see for example U.S.Pat. No. 4,214,060. Generally, fluoroelastomers are produced in anemulsion polymerization process wherein a water-soluble polymerizationinitiator and a relatively large amount of surfactant are employed. Theresulting fluoroelastomer leaves the reactor in the form of a latexwhich must be degassed (i.e. freed from unreacted monomers), coagulated,filtered and washed. Emulsion processes suffer from severaldisadvantages including production of polymers having high Mooneyviscosity, which tends to make it difficult to process these materials(i.e. mixing, extruding, molding) into cured articles, due to thepresence of ionic end groups on the fluoroelastomer polymer chains.Another disadvantage is that the polymer products contain impuritiesfrom retained surfactants, coagulants, buffers and defoamers.

On the other hand, in a suspension polymerization process,polymerization is carried out by dispersing one or more monomers, or anorganic solvent with monomer dissolved therein, in water and using anoil-soluble organic peroxide. No surfactant or buffer is required andfluoroelastomer is produced in the form of polymer particles which maybe directly filtered, i.e. without the need for coagulation, and thenwashed, thus producing a cleaner polymer than that resulting from anemulsion process. Also, the fluoroelastomer polymer chains aresubstantially free of ionic end groups so that the Mooney viscosity isrelatively low and the polymer has improved processability compared topolymer produced by an emulsion process (U.S. Pat. Nos. 3,801,552,4,985,520 and 5,824,755).

A disadvantage of suspension polymerization processes disclosed in theprior art is that it is difficult to incorporate a cure site monomeruniformly into the polymer because polymerization rate and polymermolecular weight increase throughout the reaction period. Many cure sitemonomers, if present in excess, greatly hinder the polymerizationreaction, so that the desired polymerization rate and polymer molecularweight can not be attained in suspension polymerization processes of theprior art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a suspensionpolymerization process for the production of fluoroelastomers havinguniformly distributed copolymerized units of one or more cure sitemonomers. The fluoroelastomers are characterized by having molecularweights sufficiently high to permit processing and curing, usingconventional techniques.

A further aspect of the invention relates to production offluoroelastomer products which are substantially free of ionic endgroups. Such fluoroelastomers have lower Mooney viscosities thanfluoroelastomers of similar comonomer composition and molecular weightproduced from an emulsion polymerization process. By “substantially noionic end groups” is meant fewer than 1 milliequivalents of ionic endgroups per kg fluoroelastomer. Ionic (or ionizable) end groups include,but are not limited to, sulfate, sulfonate, sulfonic acid, carboxyl andcarboxylate end groups.

In particular, the present invention is directed to a suspension processfor producing a fluoroelastomer having a selected molar ratio ofcopolymerized monomer units, said fluoroelastomer comprisingcopolymerized units of vinylidene fluoride major monomer, at least oneother copolymerizable fluorinated major monomer, and at least one curesite monomer, comprising the steps of:

(A) charging a reactor with a quantity of an aqueous medium comprising asuspension stabilizer, said suspension stabilizer being present in saidaqueous medium at a concentration of 0.001 to 3 parts by weight per 100parts of said aqueous medium; said quantity of aqueous medium being suchthat a sufficient vapor space is left in said reactor for receivinggaseous monomer;

(B) charging the vapor space in said reactor with an initial quantity ofa gaseous monomer mixture comprising vinylidene fluoride major monomerand at least one other fluorinated major monomer; and continuouslymixing said aqueous medium and said monomer mixture to form adispersion;

(C) initiating polymerization of said monomers at a temperature of 45°C. to 70° C. by adding to said dispersion an oil soluble organicperoxide polymerization initiator in an amount of 0.001 to 5 parts byweight per 100 parts of said aqueous medium, said initiator being addedas a solution consisting essentially of 0.1 to 75 wt. % of an oilsoluble organic peroxide in a water-soluble hydrocarbon solvent; and

(D) incrementally feeding to said reactor, during polymerization, so asto maintain a constant pressure in said reactor, said major monomers andat least one cure site monomer, said major monomers and said cure sitemonomer being fed to the reactor in said selected molar ratio until afluoroelastomer product having a number average molecular weight ofbetween 50,000 to 2,000,000 daltons is obtained.

Optionally, a chain transfer agent may be added near the beginning ofthe polymerization process and additional quantities may be introducedthroughout the process.

Another embodiment of this invention is the fluoroelastomer produced bythe above process of this invention. Such a fluoroelastomer may bedistinguished from a fluoroelastomer made by a different polymerizationprocess in that fluoroelastomers of this invention i) are substantiallyfree of ionic end groups (as defined above), ii) contain polymer chainend groups derived from an oil-soluble organic peroxide polymerizationinitiator, and iii) contain copolymerized units of a cure site monomer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a suspension polymerization processfor producing a fluoroelastomer which contains copolymerized units ofvinylidene fluoride (VF₂), units of at least one otherfluorine-containing copolymerizable major monomer, and units of at leastone cure site monomer. By “major monomer” is meant any copolymerizablemonomer other than a cure site monomer. The resulting fluoroelastomerhas a lower Mooney viscosity and fewer ionic end groups than does afluoroelastomer of the same monomer composition and molecular weightthat is produced from an emulsion polymerization process.Fluoroelastomers produced by the suspension polymerization process ofthis invention have improved processability (i.e. improvedextrudability, ease of mixing, moldability and demolding).

According to the present invention, fluorine-containing major monomerscopolymerizable with VF₂ include, but are not limited to,hexafluoropropylene (HFP), tetrafluoroethylene (TFE),chlorotrifluoroethylene (CTFE) and a perfluoro(alkyl vinyl) ether(PAVE).

Perfluoro(alkyl vinyl ethers) (PAVE) suitable for use as monomersinclude those of the formula

CF₂═CFO(R_(f′)O)_(n)(R_(f″)O)_(m)R_(f)  (I)

where R_(f′) and R_(f″) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R_(f) is a perfluoroalkyl group of 1-6 carbon atoms.

A preferred class of perfluoro(alkyl vinyl ethers) includes compositionsof the formula

CF₂═CFO(CF₂CFXO)_(n)R_(f)  (II)

where X is F or CF₃, n is 0-5, and R_(f) is a perfluoroalkyl group of1-6 carbon atoms.

A most preferred class of perfluoro(alkyl vinyl ethers) includes thoseethers wherein n is 0 or 1 and R_(f) contains 1-3 carbon atoms. Examplesof such perfluorinated ethers include perfluoro(methyl vinyl ether)(PMVE) and perfluoro(propyl vinyl ether) (PPVE). Other useful monomersinclude compounds of the formula

CF₂═CFO[(CF₂)_(m)CF₂CFZO]_(n)R_(f)  (III)

where R_(f) is a perfluoroalkyl group having 1-6 carbon atoms,

m=0 or 1, n=0-5, and Z=F or CF₃.

Preferred members of this class are those in which R_(f) is C₃F₇, m=0,and n=1.

Additional perfluoro(alkyl vinyl ether) monomers include compounds ofthe formula

CF₂═CFO[(CF₂CF{CF₃}O)_(n)(CF₂CF₂CF₂O)_(m)(CF₂)_(p)]C_(x)F_(2x+1)  (IV)

where m and n independently=0-10, p=0-3, and x=1-5.

Preferred members of this class include compounds where n=0-1, m=0-1,and x=1.

Additional examples of useful perfluoro(alkyl vinyl ethers) include

CF₂═CFOCF₂CF(CF₃)O(CF₂O)_(m)C_(n)F_(2n+1)  (V)

where n=1-5, m=1-3, and where, preferably, n=1.

PAVE-containing fluoroelastomers of the invention contain between 23 and65 wt. % copolymerized VF₂ units, preferably between 30 and 65 wt. % ofsuch units. If less than 23 wt. % vinylidene fluoride units are present,the polymerization rate is very slow. In addition, good low temperatureflexibility cannot be achieved. Vinylidene fluoride levels above 65 wt.% result in polymers that contain crystalline domains and arecharacterized by poor low temperature compression set resistance andreduced fluids resistance.

The PAVE content of the PAVE-containing fluoroelastomers of theinvention ranges from 25 to 75 wt. %. If perfluoro(methyl vinyl ether)is used, then the fluoroelastomer preferably contains between 30 and 40wt. % copolymerized PMVE units. If less than 25 wt. % perfluoro(alkylvinyl ether) is present, the low temperature properties of thefluoroelastomers are adversely affected.

Copolymerized units of tetrafluoroethylene may also be present in thePAVE-containing fluoroelastomers of the invention at levels up to 30 wt.%. The presence of copolymerized units of TFE is desirable for thepurpose of increasing fluorine content without unduly compromising lowtemperature flexibility. High fluorine content promotes good fluidresistance. If TFE is present as a monomer, it is preferablycopolymerized in amounts of at least 3 wt. %. Levels of 3 wt. % orgreater TFE lead to improved fluid resistance in some end useapplications. TFE levels above 30 wt. % result in some polymercrystallinity which affects low temperature compression set andflexibility.

Fluoroelastomers containing units of PAVE are especially preferred inthe present invention because of the combination of good low temperaturesealing properties and good fluid resistance of these curedfluoroelastomers. Also, when 2-hydropentafluoropropylene cure sitemonomer is incorporated into PAVE-containing fluoroelastomers made bythe suspension process of this invention, the fluoroelastomers showenhanced polyol curability compared to PAVE-containing fluoroelastomersmade by an emulsion process.

The fluoroelastomers of the present invention also comprise units of oneor more cure site monomers. Examples of suitable cure site monomersinclude: 2-hydropentafluoropropylene (2-HPFP, also referred to in theart as 1,1,3,3,3-pentafluoropropene); a non-conjugated diene (resultingin a reactive double bond cure site); a bromine- or iodine-containingolefin; and a bromine- or iodine-containing unsaturated ether. Units ofcure site monomer are typically present in fluoroelastomers at a levelof 0.3-7 wt. %, preferably 0.5-5 wt. % and most preferably between 0.7and 3 wt %.

Brominated cure site monomers may contain other halogens, preferablyfluorine. Examples of brominated olefin cure site monomers arebromotrifluoroethylene; 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); andothers such as vinyl bromide, 1-bromo-2,2-difluoroethylene;perfluoroallyl bromide; 4-bromo-1,1,2-trifluorobutene;4-bromo-1,1,3,3,4,4, -hexafluorobutene;4-bromo-3-chloro-1,1,3,4,4-pentafluorobutene;6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1 and3,3-difluoroallyl bromide. Brominated unsaturated ether cure sitemonomers useful in the invention include 2-bromo-perfluoroethylperfluorovinyl ether and fluorinated compounds of the classCF₂Br—R_(f)—O—CF═CF₂, such as CF₂BrCF₂O—CF═CF₂, and fluorovinyl ethersof the class ROCF═CFBr or ROCBr═CF₂, where R is a lower alkyl group orfluoroalkyl group, such as CH₃OCF═CFBr or CF₃CH₂ OCF═CFBr.

Suitable iodinated cure site monomers include iodinated olefins of theformula: CHR═CH—Z—CH₂CHR—I, wherein R is —H or —CH₃; Z is a C₁-C₁₈(per)fluoroalkylene radical, linear or branched, optionally containingone or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radicalas disclosed in U.S. Pat. No. 5,674,959. Other examples of usefuliodinated cure site monomers are unsaturated ethers of the formula:I(CH₂CF₂CF₂)_(n)OCF═CF₂ and ICH₂CF₂O[CF(CF₃)CF₂]_(n) CF═CF₂, and thelike, wherein n=1-3, such as disclosed in U.S. Pat. No. 5,717,036. Inaddition, suitable iodinated cure site monomers including iodoethylene,4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB);3-chloro-4-iodo-3,4,4-trifluorobutene; 2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane;2-iodo-1-(perfluorovinyloxy)-1,1,2,2-tetrafluoroethylene;1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethylvinyl ether; 3,3,4,5,5,5-hexafluoro-4-iodopentene; andiodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyliodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also usefulcure site monomers.

Examples of non-conjugated diene cure site monomers include1,4-pentadiene, 1,5-hexadiene, 1,7-octadiene and others, such as thosedisclosed in Canadian Patent 2,067,891. A suitable triene is8-methyl-4-ethylidene-1,7-octadiene.

Of the cure site monomers listed above, preferred compounds, forsituations wherein the fluoroelastomer will be cured with peroxide,include 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB);4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); allyl iodide; andbromotrifluoroethylene. When the fluoroelastomer will be cured with apolyol, 2-HPFP is the preferred cure site monomer.

Additionally, iodine-containing end groups, bromine-containing endgroups or mixtures thereof may optionally be present at one or both ofthe fluoroelastomer polymer chain ends as a result of the use of chaintransfer or molecular weight regulating agents during preparation of thefluoroelastomers. The amount of chain transfer agent, when employed, iscalculated to result in an iodine or bromine level in thefluoroelastomer in the range of 0.005-5 wt. %, preferably 0.05-3 wt. %.

Examples of chain transfer agents include iodine-containing compoundsthat result in incorporation of bound iodine at one or both ends of thepolymer molecules. Methylene iodide; 1,4-diiodoperfluoro-n-butane; and1,6-diiodo-3,3,4,4, tetrafluorohexane are representative of such agents.Other iodinated chain transfer agents include1,3-diiodoperfluoropropane; 1,4-diiodoperfluorobutane;1,6-diiodoperfluorohexane; 1,3-diiodo-2-chloroperfluoropropane;1,2-di(iododifluoromethyl)-perfluorocyclobutane;monoiodoperfluoroethane; monoiodoperfluorobutane;2-iodo-1-hydroperfluoroethane, etc. Particularly preferred arediiodinated chain transfer agents.

Examples of brominated chain transfer agents include1-bromo-2-iodoperfluoroethane; 1-bromo-3-iodoperfluoropropane;1-iodo-2-bromo-1,1-difluoroethane and others such as disclosed in U.S.Pat. No. 5,151,492.

Cure site monomers and chain transfer agents are typically added to thereactor as liquid solutions in the same solvent that is employed for theoil-soluble peroxide polymerization initiator (described below). Inaddition to being introduced into the reactor near the beginning ofpolymerization, quantities of chain transfer agent may be addedthroughout the entire polymerization reaction period, depending upon thedesired composition of the fluoroelastomer being produced, the chaintransfer agent being employed, and the total reaction time.

In the suspension polymerization process of this invention, (1) agaseous monomer mixture of a desired composition (initial monomercharge) is introduced into the vapor space above an aqueous medium in areactor. The aqueous medium comprises a suspension stabilizer in anamount of 0.001-3 parts (by weight) per 100 parts (by weight) of theaqueous medium. The monomer mixture is then dispersed in the aqueousmedium and, optionally, a chain transfer agent is also added while thereaction mixture is agitated, typically by mechanical stirring. In theinitial gaseous monomer charge, the relative amount of each monomer isdictated by reaction kinetics and is set so as to result in afluoroelastomer having the desired (i.e. selected) molar ratio ofcopolymerized monomer units (e.g. very slow reacting monomers must bepresent in a higher molar amount relative to the other monomers than isdesired in the composition of the fluoroelastomer to be produced); (2)the temperature of the reaction mixture is maintained in the range of45° C.-70° C., preferably 50° C.-60° C.; (3) the suspensionpolymerization reaction is then initiated by adding an oil-solubleorganic peroxide in an amount so as to result in between 0.001 and 5parts by weight peroxide per 100 parts by weight of the aqueous medium.The peroxide is added as a solution consisting essentially of anoil-soluble organic peroxide in a water-soluble hydrocarbon solvent.Depending upon the nature of the fluoroelastomer to be produced and thetotal polymerization time, it may be necessary to add additionalperoxide initiator to the reactor during the course of polymerization inorder to keep the level of peroxide within the above range; and (4)additional quantities of the gaseous major monomers and cure sitemonomer (incremental feed) are added at a controlled rate throughout thepolymerization in order to maintain a constant reactor pressure at acontrolled temperature. Since polymerization rate constantly increasesover the course of the reaction period, the flow rate of gaseous majormonomer and cure site monomer must be increased over the course of thereaction period in order to maintain constant pressure within thereactor. The relative amount (i.e. molar ratio) of both the gaseousmajor monomers and cure site monomer in the incremental feed isapproximately the same as the selected molar ratio of copolymerizedmonomer units in the fluoroelastomer to be prepared. The amount ofpolymer formed is approximately equal to the cumulative amount ofincremental monomer feed. One skilled in the art will recognize that themolar ratio of monomers in the incremental feed is not necessarilyexactly the same as that of the desired (i.e. selected) copolymerizedmonomer unit composition in the resulting fluoroelastomer because thecomposition of the initial charge may not be exactly that required forthe selected final fluoroelastomer composition, or because a portion ofthe monomers in the incremental feed may dissolve into the polymerparticles already formed, without reacting. However, in practice, thecompositions of the initial charge and the incremental feed are oftenvery similar to each other and to the composition of copolymerizedmonomer units desired in the fluoroelastomer to be produced.Polymerization times in the range of from 3 to 50 hours are employed inthis invention.

The polymerization temperature is maintained in the range of 45° C.-70°C. If the temperature is below 45° C., the rate of polymerization is tooslow for efficient reaction on a commercial scale, while if thetemperature is above 70° C., suspended particles of the fluoroelastomercopolymer formed become sticky and are liable to cause plugging in thepolymerization reactor and make it difficult to maintain a stable stateof suspension during the polymerization reaction.

The polymerization pressure is in the range of 0.7 to 3.5 MPa,preferably 1.0 to 2.5 MPa. The desired polymerization pressure isinitially achieved by adjusting the amount of gaseous monomers in theinitial charge, and after the reaction is initiated, the pressure isadjusted by controlling the incremental gaseous monomer feed. Thepolymerization pressure is set in the above range because if it is below0.7 MPa, the monomer concentration in the polymerization reaction systemis too low to obtain a satisfactory reaction rate. In addition, themolecular weight does not increase sufficiently. If the pressure isabove 3.5 MPa, the amount of monomer liquefied in the reactor isincreased, thereby merely increasing the amount of monomer which is notconsumed, resulting in poor production efficiency.

It is very important that cure site monomers other than 2H-PFP, not bepresent prior to initiation of the polymerization reaction. It is alsoimportant that cure site monomer introduced in the incremental feed notbe added to the reactor in an excess molar amount. Otherwise,polymerization is terminated early and only low molecular weightfluoroelastomers are produced. The fluoroelastomers produced by thisinvention have number average molecular weights in the range of about50,000 to 2,000,000 daltons. To obtain fluoroelastomers of suchmolecular weights having uniform composition of copolymerized units ofmajor monomers and cure site monomer, cure site monomer must be addedwith the incremental feed in a set ratio equal to the selected level tobe incorporated into the polymer. Cure site comononer addition iscontrolled such that the ratio of cure site monomer to total incrementalmonomer feed is in the range of 0.3 wt. % to 7 wt. %, preferably in therange of 0.5 wt. % to 5 wt. %.

It is relatively easy to control the flow rate of incremental feedgaseous major monomers in order to maintain constant pressure within thereactor throughout the entire polymerization reaction period. However,controlling the flow rate of the liquid cure site monomer solution maybe problematic. In the case of the gaseous major monomers, a pressurecontroller may simply increase the flow rate of gaseous monomer to thereactor in order to maintain constant pressure within the reactor aspolymerization rate increases. In the early stages of the polymerizationreaction, when the polymerization rate is low, and flow rate of gaseousmajor monomer incremental feed is very small, it may be necessary to usea gas accumulator between the reactor and major monomer source in orderto accurately control the flow rate and thus maintain constant pressurewithin the reactor.

The flow rate of gaseous major monomer must be maintained in a setproportion to the flow rate of liquid cure site monomer solutionthroughout the entire reaction in order to produce a fluoroelastomer ofuniform composition having the selected molar ratios of copolymerizedunits of major monomers and cure site monomer. Therefore as the flowrate of gaseous major monomers is increased during the reaction period,the flow rate of the liquid cure site monomer solution must besimultaneously increased by a proportional amount. One skilled in theart will readily recognize several means to accomplish this. Forexample, a flow rate monitor may be placed in the gascous monomerincremental feed line and then the flow rate of the liquid cure sitemonomer can be increased proportionally, either manually orautomatically, as the flow rate of gaseous monomer is increased.Alternatively, the average flow rate of gaseous major monomer may bedetermined over several discrete time periods throughout the totalreaction time. The flow rate of the cure site monomer can then be setproportionally to the average gaseous monomer flow rate during eachdiscrete time period.

The amount of fluoroelastomer copolymer formed is approximately equal tothe amount of incremental feed charged, and is in the range of 10-300parts by weight of copolymer per 100 parts by weight of aqueous medium,preferably in the range of 20-250 parts by weight of the copolymer.

The degree of copolymer formation is set in the above range because ifit is less than 10 parts by weight, productivity is undesirably low,while if it is above 300 parts by weight, the solids content becomes toohigh for satisfactory stirring.

Oil-soluble organic peroxides which may be used to initiatepolymerization in this invention include, for example,dialkylperoxydicarbonates, such as diisopropylperoxydicarbonate (IPP),di-sec-butylperoxydicarbonate, di-sec-hexylperoxydicarbonate,di-n-propylperoxydicarbonate, and di-n-butyl peroxydicarbonate;peroxyesters, such as tert-butylperoxyisobutyrate andtert-butylperoxypivalate; diacylperoxides, such as dipropionyl peroxide;and di(perfluoroacyl)peroxides or di(chlorofluoroacyl)peroxides such asdi(perfluoropropionyl)pcroxide anddi(trichloro-octafluorohexanoyl)peroxide. The use ofdialkylperoxydicarbonates is preferable, and the use of IPP is mostpreferred. These oil soluble organic peroxides may be used alone or as amixture of two or more types. The amount to be used is selectedgenerally in the range of 0.001-5 parts by weight per 100 parts byweight of the aqueous medium, preferably 0.01-3 parts by weight. Duringpolymerization some of the fluoroelastomer polymer chain ends are cappedwith fragments generated by the decomposition of these peroxides.

In the suspension polymerization process of the invention, theoil-soluble organic peroxide is added to the reactor as a solutionconsisting essentially of 0-75 wt % (preferably 1-60 wt. %) peroxide ina water-soluble hydrocarbon solvent. If the concentration of peroxide isover 75 wt %, the organic peroxide concentration is too high for safetransportation. On the other hand, if it is below 0.1 wt %, theconcentration is so low that the amount of solvent to be recovered afterpolymerization becomes undesirably high.

The water-soluble hydrocarbon solvent contains no halogen atoms and isrepresented by the general formulas R₁OH, R₂COOR₁, or R₁COR₃, where R₁and R₃ are methyl or t-butyl groups, and R₂ is hydrogen, a methyl groupor a t-butyl group. The hydrocarbon solvents useful in the presentinvention do not have substantial adverse effects on the polymerizationreaction because the chain transfer reactivity of these hydrocarbonsolvents is relatively small. At the same time, they are soluble in theaqueous reactor medium. Further, only small amounts are contained indroplets comprised of the monomers and oil soluble organic peroxide inwhich the polymerization reaction occurs. Also, polymerizationconditions are set so that both solvent and monomer concentrations arelow (generally less that 10 wt. %) in the fluoroelastomer copolymerformed in the reactor. Thus recovery of the solvent and monomers is notdifficult.

Specific examples of water-soluble, non-halogenated hydrocarbon solventsuseful in this invention are methanol, tert-butyl alcohol, methylformate, tert-butyl formate, methyl acetate, tert-butyl acetate, methylpivalate, tert-butyl pivalate, acetone, methyl tert-butyl ketone, anddi-tert-butyl ketone. The use of methanol, tert-butyl alcohol, methylacetate, or tert-butyl acetate is preferable. Methyl acetate ortert-butyl acetate are most preferred. These solvents may be used aloneor as a combination of two or more types.

Suspension stabilizers useful in the present invention include, forexample, methyl cellulose, carboxymethyl cellulose, bentonite, talc, anddiatomaceous earth. Methyl cellulose is preferred. Typically the numberaverage molecular weight of the methyl cellulose is between 15,000 and70,000. These suspension stabilizers may be used alone or as acombination of two or more types. The amount utilized is generally inthe range of 0.001-3 parts by weight, preferably 0.01-1 part by weightper 100 parts by weight of the aqueous medium.

The monomer composition of the initial charge and that of theincremental feed are determined by gas chromatography. The monomercomposition (i.e. the mole percentage of copolymerized monomer units) inthe fluoroelastomer copolymer prepared is determined by dissolving thefluoroelastomer in deutero-acetone and carrying out ¹H and ¹⁹F-NMRanalysis, or by FTIR analysis of thin films. X-ray fluorescence is usedto determine concentrations of bromine- and iodine-containing curesites.

Another embodiment of this invention is the novel fluoroelastomersproduced by the above suspension process of this invention. Suchfluoroelastomers may be distinguished from fluoroelastomers made by adifferent polymerization process in that fluoroelastomers of thisinvention i) are substantially free of ionic endgroups (as defined abovein the Summary of the Invention), ii) contain polymer chain endgroupsderived from an oil-soluble organic peroxide polymerization initiator,and iii) contain copolymerized units of a cure site monomer.Emulsion-produced fluoroelastomers of similar copolymerized monomer unitcomposition as the fluoroelastomers of this invention will either havemore than one milliequivalent of ionic endgroups per kg offluoroelastomer, or the emulsion-produced fluoroelastomer will haveendgroups derived from a water-soluble inorganic peroxide polymerizationinitiator (such as ammonium persulfate), or both.

The fluoroelastomers prepared by this invention are generally molded andvulcanized during fabrication into finished products such as seals, wirecoatings, hose, etc. Suitable vulcanization methods employ polyol,polyamine, or organic peroxide compounds as curatives. Vulcanizationwith a polyol compound is especially advantageous because compressionset resistance of the cured fluoroelastomer is generally better thanthat obtained using polyamine or peroxide curatives. Polyol curativesare particularly effective curatives for fluoroelastomers which containcopolymerized units of 2-HPFP cure site monomer, in particularfluoroelastomers comprising copolymerized units of 30-65 wt. % VF₂,30-40 wt. % PMVE, 3-30 wt. % TFE and 0.5-3 wt. % 2-HPFP.

When peroxide curatives are used, resistance of the curedfluoroelastomers to chemicals such as acids or bases is markedlyimproved. Peroxide curatives are particularly useful for vulcanizingfluoroelastomers which contain either a bromine-, or iodine-containingcure site monomer. The latter type of fluoroelastomers which also haveiodine or bromine at one or more polymer chain ends cure especially wellwith peroxide curatives. PMVE-containing fluoroelastomers comprisingcopolymerized units of 30-65 wt. % VF₂, 30-40 wt. % PMVE, 3-30 wt. % TFEand 0.5-3 wt. % of either BTFB, ITFB or allyl iodide are preferredperoxide-curable polymers. Fluoroelastomers containing copolymerizedunits of 30-65 wt. % VF₂, 25-40 wt. % HFP, 3-30 wt. % TFE and 0.5-3 wt.% of either BTFB, ITFB or allyl iodide are also preferredperoxide-curable polymers.

Any of the known polyol aromatic crosslinking agents that requireaccelerators for satisfactory cure rates are suitable for use with thefluoroelastomers prepared by the present invention. The crosslinkingagent is usually added in amounts of from about 0.5-4 parts by weightper hundred parts by weight fluoroelastomer (phr), usually 1-2.5 phr.Preferred crosslinking agents are di- tri-, tetrahydroxybenzenes,naphthalenes, anthracenes and bisphenols of the formula

where A is a stable divalent radical, such as a difunctional aliphatic,cycloaliphatic, or aromatic radical of 1-13 carbon atoms, or a thio,oxy, carbonyl, sulfinyl, or sulfonyl radical; A is optionallysubstituted with at least one chlorine or fluorine atom; x is 0 or 1; nis 1 or 2 and any aromatic ring of the polyhydroxylic compound isoptionally substituted with at least one atom of chlorine, fluorine, orbromine, a —CHO group, or a carboxyl or acyl radical (e.g. a —COR whereR is OH or a C₁-C₈ alkyl, aryl, or cycloalkyl group). It will beunderstood from the above formula describing bisphenols that the —OHgroups can be attached in any position (other than number one) in eitherring. Blends of two or more such compounds can also be used.

Referring to the bisphenol formula shown in the previous paragraph, whenA is alkylene, it can be, for example, methylene, ethylene,chloroethylene, fluoroethylene, difluoroethylene, 1,3-propylene,1,2-propylene, tetramethylene, chlorotetramethylene,fluorotetramethylene, trifluorotetramethylene, 2-methyl-1,3-propylene,2-methyl-1,2-propylene, pentamethylene, and hexamethylene. When A isalkylidene, it can be for example ethylidene, dichloroethylidene,difluoroethylidene, propylidene, isopropylidene,trifluoroisopropylidene, hexafluoroisopropylidene, butylidene,heptachlorobutylidene, heptafluorobutylidene, pentylidene, hexylidene,and 1,1-cyclohexylidene. When A is a cycloalkylene radical, it can befor example 1,4-cyclohexylene; 2-chloro-1,4-cyclohexylene;2-fluoro-1,4-cyclohexylene; 1,3-cyclohexylene; cyclopentylene;chlorocyclopentylene; fluorocyclopentylene; and cycloheptylene. Further,A can be an arylene radical such as m-phenylene; p-phenylene;2-chloro-1,4-phenylene; 2-fluoro-1,4-phenylene; o-phenylene;methylphenylene; dimethylphenylene; trimethylphenylene;tetramethylphenylene; 1,4-naphthylene; 3-fluoro-1,4-naphthylene;5-chloro-1,4-naphthylene; 1,5-naphthylene; and 2,6-naphthylene.

Other useful crosslinking agents include hydroquinone, dihydroxybenzenessuch as catechol, resorcinol, 2-methyl resorcinol, 5-methyl resorcinol,2-methyl hydroquinone, 2,5-dimethyl hydroquinone; 2-t-butylhydroquinone; and 1,5-dihydroxynaphthalene.

Additional polyol curing agents include alkali metal salts of bisphenolanions, quaternary ammonium salts of bisphenol anions and quaternaryphosphonium salts of bisphenol anions. For example, the salts ofbisphenol A and bisphenol AF. Specific examples include the disodiumsalt of bisphenol AF, the dipotassium salt of bisphenol AF, themonosodium monopotassium salt of bisphenol AF and thebenzyltriphenylphosphonium salt of bisphenol AF. Quaternary ammonium andphosphonium salts of bisphenol anions and their preparation arediscussed in U.S. Pat. Nos. 4,957,975 and 5,648,429.

In addition, derivatized polyol compounds, such as diesters, are usefulcrosslinking agents. Examples of such compositions include diesters ofphenols, such as the diacetate of bisphenol AF, the diacetate ofsulfonyl diphenol, and the diacetate of hydroquinone.

When cured with polyol compounds, the curable compositions will alsogenerally include a cure accelerator. The most useful accelerators arequaternary phosphonium salts, quaternary alkylammonium salts, ortertiary sulfonium salts. Particularly preferred accelerators aren-tetrabutylammonium hydrogen sulfate, tributylallylphosphonium chlorideand benzyltriphenylphosphoniunm chloride. Other useful acceleratorsinclude those described in U.S. Pat. Nos. 5,591,804; 4,912,171;4,882,390; 4,259,463 and 4,250,278 such as tributylbenzylammoniumchloride, tetrabutylammonium bromide, tetrabutylammonium chloride,benzyl tris(dimethylamino)phosphonium chloride;8-benzyl-1,8-diazabicyclo[5,4,0]-7-undecenonium chloride,[(C₆H₅)₂S⁺(C₆H₁₃)][Cl]³¹ , and [(C₆H₁₃)₂S(C₆H₅)]⁺[CH₃CO₂]⁻. In general,about 0.2 phr accelerator effective amount, and preferably about0.35-1.5 phr is used.

If quaternary ammonium or phosphonium salts of bisphenols are used ascuring agents, then addition of a cure accelerator is not necessary.

The polyol cure system will also contain a metal compound composed of adivalent metal oxide, such as magnesium oxide, zinc oxide, calciumoxide, or lead oxide, or a divalent metal hydroxide; or a mixture of theoxide and/or hydroxide with a metal salt of a weak acid, for example amixture containing about 1-70 percent by weight of the metal salt. Amongthe useful metal salts of weak acids are barium, sodium, potassium,lead, and calcium stearates, benzoates, carbonates, oxalates, andphosphites. The amount of the metal compound added is generally about1-15 phr, about 2-10 parts being preferred.

Polyamines and diamine carbamates are also useful curing agents for thecompositions of the invention. Examples of useful polyamines includeN,N′-dicinnamylidene-1,6-hexanediamine, trimethylenediamine,cinnamylidene trimethylenediamine, cinnamylidene ethylenediamine, andcinnamylidene hexamethylenediamine. Examples of useful carbamates arehexamethylenediamine carbamate, bis(4-aminocyclohexyl)methane carbamate,1,3-diaminopropane monocarbamate, ethylenediamine carbamate andtrimethylenediamine carbamate. Usually about 0.1-5 phr of the carbamateis used.

The peroxide vulcanization method can be exemplified as follows. To afluoroelastomer prepared by this invention is added (a) an organicperoxide, (b) a polyfunctional unsaturated compound, and (c) a divalentmetal hydroxide, a divalent metal oxide, or a combination of the two.

Organic peroxides suitable for use include:1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane;1,1-bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane;n-butyl-4,4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane;2,5-dimethylhexane-2,5-dihydroxyperoxide; di-t-butyl peroxide;t-butylcumyl peroxide; dicumyl peroxide; alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene;2,5-dimethyl-2,5-di(t-butylperoxy)hexane;2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide,t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane;t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferredexamples of the component organic peroxides include2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and alpha,alpha′-bis(t-butylperoxy-m-isopropyl)benzene. The amount compounded isgenerally in the range of 0.05-5 parts by weight, preferably in therange of 0.1-3 parts by weight per 100 parts by weight of thefluoroelastomer. This particular range is selected because if theperoxide is present in an amount of less than 0.05 parts by weight, thevulcanization rate is insufficient and causes poor mold release. On theother hand, if the peroxide is present in amounts of greater than 5parts by weight, the compression set of the cured polymer becomesunacceptably high. In addition, the organic peroxides may be used singlyor in combinations of two or more types.

Specific examples of the polyfunctional unsaturated compound used in theperoxide vulcanization method are triallyl cyanurate, trimethacrylisocyanurate, triallyl isocyanurate, trimethallyl isocyanurate, triacrylformal, triallyl trimellitate, N,N′-m-phenylene bismaleimide, diallylphthalate, tetraallylterephthalamide, tri(diallylamine)-s-triazine,triallyl phosphite, and N,N-diallylacrylamide. The amount compounded isgenerally in the range of 0.1-10 parts by weight per 100 parts by weightof the fluoroelastomer. This particular concentration range is selectedbecause if the unsaturated compound is present in amounts less than 0.1part by weight, crosslink density of the cured polymer is unacceptable.On the other hand, if the unsaturated compound is present in amountsabove 10 parts by weight, it blooms to the surface during molding,resulting in poor mold release characteristics. The preferable range ofunsaturated compound is 0.2-6 parts by weight per 100 partsfluoroelastomer. The unsaturated compounds may be used singly or as acombination of two or more types.

In addition, if necessary, other components, for example, fillers suchas carbon black, Austin black, graphite, thermoplastic fluoropolymermicropowders, silica, clay, diatomaccous earth, talc, wollastonite,calcium carbonate, calcium silicate, calcium fluoride, and bariumsulfate; processing aides such as higher fatty acid esters, fatty acidcalicum salts, fatty acidamides (e.g. erucamide), low molecular weightpolyethylene, silicone oil, silicone grease, stearic acid, sodiumstearate, calicium stearate, magnesium sterate, aluminum stearate, andzinc stearate, coloring agents such as titanium white and iton red maybe used as compound additives. The amount of such filler compounded isgenerally in the range of 0.1-100 parts by weight, preferably 1-60 partsby weight, per 100 parts by weight of the flouroelaster. This range isselected because if the filler is present in amounts of less than 0.1part by weight, there is little or no effect, while, on the other hand,if greater than 100 parts by weight are used, elasticity is sacrificed.The amount of processing aid compounded is generally less than 10 partsby weight, preferably less than 5 parts by weight, per 100 parts byweight of the fluorlastomer. If the amount used is above the limit, heatresistance is adversely affected. The amount of a coloring agentcompounded is generally less than 50 parts per weight, preferaably lessthan 30 parts per weight per 100 parts by weight of the fluorelastomer.If greater than 50 parts by weight is used, compression set suffers.

The fluoroelastomers prepared by the process of this invention areuseful in many industrial applications including seals, wire coatings,tubing and laminates.

EXAMPLES Test Methods

Limiting viscosity [η] was determined by dissolving a sample of thefluoroelastomer in methyl ethyl ketone to obtain a 0.1 g/100 mlsolution, which was used in a capillary viscometer in measurements at30° C.

Mooney viscosity, ML (1+10), was determined according to ASTM D1646 withan L (large) type rotor at 121° C., using a preheating time of oneminute and rotor operation time of 10 minutes.

Cure Characteristics

Cure characteristics were measured, as indicated in the examples, usingeither an Alpha Technologies Ltd. oscillating disk rheometer (ODR) 2000Einstrument, under conditions corresponding to ISO 3417 at 1° arc, 24minutes, 180° C., sample size of 13-15 g; or an Alpha Technologies Ltd.MDR 2000E instrument under the following conditions: ISO 6502 at movingdie frequency of 1.66 Hz, oscillation amplitude of ±0.5 degrees,temperature of 180° C., sample size of 7-8 g, and the duration of thetest was 12 minutes. The following cure parameters were recorded:

M_(H): maximum torque level, in units of dN•m

M_(L): minimum torque level, in units of dN•m

t_(s)2: minutes to a 2.26 dNm rise above M_(L)

t50: minutes to 50% of maximum torque

t90: minutes to 90% of maximum torque

Tensile Properties

Unless otherwise noted, stress/strain properties were measured on testspecimens that had been press cured at 180° C. for 10 minutes and thenpost cured in a hot air oven for 24 hours at 200° C. The followingphysical property parameters were recorded; test methods are inparentheses:

M₁₀₀: modulus at 100% elongation in units of MPa (ISO 37)

T_(B): tensile strength in units of MPa (ISO 37)

E_(B): elongation at break in units of % (ISO 37)

T_(g): glass transition temperature was measured by differentialscanning calorimetry using a 10°C./minute heating rate.

Hardness (Shore A, ISO 868)

Compression set of small pip samples (ISO 815)

The invention is further illustrated by, but is not limited to, thefollowing examples.

A polymerization reactor was configured for carrying out semibatchpolymerizations. The gaseous monomer feed system and its operationconsisted of a source line for each gaseous major monomer, flowcontroller for each gaseous monomer line, a compressor, an accumulatorand a pressure controller between the accumulator and reactor vessel. Atthe beginning of the polymerization reaction, monomers were consumed inthe reactor at a low rate. The monomer supply to the compressor wassignificantly larger in order to maintain an accurate monomer feedcomposition. The difference in the amount of materials fed to thecompressor and the amount consumed in the reactor was stored in theaccumulator, placed between the reactor and the compressor. The storagein the accumulator was controlled with a pressure controller, which wascascaded to several flow controllers metering the monomer mixture to thecompressor. As monomers flowed into the accumulator, the pressureincreased to a high preset limit. When the high limit was reached, theflow controllers closed the gaseous monomer supply valves. As monomersflowed into the reactor, the accumulator pressure dropped to a lowlimit. At the low limit, the monomer supply valves opened and compressedgases were fed into the accumulator until pressure reached the high setlimit, which shut down the monomer feed. This cycle continued until thepolymerization was terminated. An exponential digital filter was used tocalculate the average flow rate of gaseous monomers during each periodthat the gaseous monomer valves were in the open position. Thecalculated average gaseous monomer flow rates were used to adjust theflow rate of the metering pump employed to pump liquid cure site monomersolution to the reactor during the same time periods.

Example 1

A semibatch suspension polymerization of this invention was carried outin a 40-liter reactor in order to produce Sample 1. The reactor wascharged with 20 liters of an aqueous solution containing 0.07 wt. % (14g) of methyl cellulose (M_(n) approximately 17,000). The air in thevapor space was replaced first with nitrogen and then with 3500 grams ofa monomer mixture (initial feed) containing 5.0 wt. %tetrafluoroethylene (TFE), 40.4 wt. % vinylidene fluoride (VF₂), and54.6 wt. % perfluoro(methyl vinyl ether) (PMVE). The reactor contentswere heated to 50° C. and pressurized with the above monomer mixture to2.76 MPa. After reaching 2.76 MPa, the reactor pressure controller wasset to operate automatically with a pressure target of 2.76 MPa.

During the monomer addition step, two solutions were prepared for thepolymerization phase: a 45 wt. % solution of methylene iodide (CH₂I₂) inmethyl acetate, and an initiator solution containing 20 wt. %diisopropyl peroxydicarbonate (IPP) in methyl acetate. The specificgravity of the iodide solution was 1.34. A liquid cure site monomer,4-bromo-3,3,4,4, -tetrafluorobutene-1 (BTFB) was also used in thepolymerization, with a goal of 0.6% incorporation in the polymer.

While holding the reactor pressure at 2.76 MPa and 50° C., theaccumulator was pressurized to 3.10 MPa with an incremental feed gaseousmonomer mixture comprising 15.1 wt. % TFE, 48.5 wt. % VF₂, and 36.8 wt.% PMVE. Once the accumulator reached 3.10 MPa, it was set to operateautomatically off the pressure controller between 3.10 MPa and 3.03 MPa.At 3.10 MPa the monomer feed valves closed, and at 3.03 MPa the monomerfeed valves were opened.

A quantity (14.5 ml) of the 45 wt. % methylene iodide solution(equivalent to 8.75 grams of CH₂I₂) was added to the reactor. After afew minutes, 50 grams of initiator solution (equivalent to 10 grams ofIPP) were added. Immediately after the initiator addition, two meteringpumps were programmed to deliver the methylene iodide, and BTFBsolutions to the reactor at rates proportional to the gaseous monomerflow rates.

An additional 26.2 grams of CH₂I₂ (43.4 ml of 45 wt. % solution) wereadded over the first 1,500 grams of total monomer consumed in thereactor. The iodide pump rate was set to deliver 2.89 ml per 100 gramsof monomer, based on the calculated average monomer flow. Similarly, theBTFB pump was set to deliver an average 0.72 grams per 100 grams ofmonomer consumed in the reactor, based on the filtered value of themonomer flow.

The initial polymerization rate was 41 grams of polymer produced/hourand it increased to 717 grams/hour after 33 hours. Two additionalportions of IPP solution were introduced to the reactor to maintain thepolymerization rate: the equivalent of 3 grams of IPP at the 17^(th)hour, and 2 grams of IPP at the 22^(nd) hour. When the polymerizationreaction was terminated, the total feed to the reactor included 15,000grams of major monomers, 15 grams of IPP, 35 grams of CH₂I₂, and 107.8grams of BTFB. Over 17 kg of polymer were produced having the propertiesshown in Table I.

TABLE I Inherent viscosity 0.91 Mooney viscosity, ML (1 + 10)(121° C.)73 TFE, wt. % 19.92 VF₂, wt. % 47.20 PMVE, wt. % 32.14 BTFB, wt. % 0.58Iodine, wt. % 0.16 Tg, ° C. −32.1

Comparative Sample A was produced by a semibatch suspensionpolymerization process similar to that described above except that curesite monomer was omitted. Polymerization was carried out in the 40-literreactor. The reactor was charged with 20 liters of an aqueous solutioncontaining 0.07 wt. % of methyl cellulose. The air in the vapor spacewas replaced first with nitrogen, then with a monomer mixture (initialfeed) containing 2.1 wt. % tetrafluoroethylene (TFE), 40.8 wt. %vinylidene fluoride (VF₂), and 57.1 wt. % perfluoro(methyl vinyl ether)(PMVE). The reactor contents were heated to 50° C. and pressurized with3328 grams of the above monomer mixture to 2.66 MPa. After the pressurereached 2.66 MPa, the reactor pressure controller was set to operateautomatically with a pressure target of 2.66 MPa.

During the monomer addition step, two solutions were prepared for thepolymerization phase: a 45 wt. % solution of methylene iodide (CH₂I₂) inmethyl acetate, and an initiator solution containing 15 wt. % IPP inmethyl acetate. The specific gravity of the iodide solution was 1.39.

A gaseous incremental monomer mixture was fed to maintain constantreactor pressure at the controlled temperature of 50° C. Incrementalfeed contained 11.1 wt. % TFE, 54.2 wt. % VF₂, and 34.7 wt. % PMVE.

A quantity (11.3 ml) of the 45 wt. % iodide solution was added to thereactor (equivalent to 7.0 grams of CH₂I₂). After a few minutes, 55.3grams of the initiator solution were added (equivalent to 8.3 grams ofIPP). Immediately after the initiator addition, a metering pump wasprogrammed to deliver the iodide solution to the reactor at ratesproportional to the gaseous monomer flow rates.

A further 28.2 grams of CH₂I₂ (45.0 ml of 45 wt. % solution) were addedin addition to the first 1,081 grams of total monomer consumed in thereactor. The iodide pump rate was set to deliver 4.50 ml per 100 gramsof gaseous major monomer, based on the calculated average gaseous majormonomer flow.

The initial polymerization rate was 14 grams of polymer produced/hourand it increased to 2000 grams/hour after 33 hours. A further additionof 4.1 grams of IPP was made at the 13^(th) hour. At the termination ofthe reaction, the total feed to the reactor included 14,009 grams ofmonomer, 12.4 grams of IPP, and 35.2 grams of CH₂I₂. Over 14.8 kg ofpolymer were produced having the properties shown in Table II.

TABLE II Inherent viscosity 0.71 Mooney viscosity, ML (1 + 10)(121° C.)32 TFE, wt. % 12.35 VF₂, wt. % 53.19 PMVE, wt. % 34.27 Iodine, 3 wt. %0.19 Tg, ° C. −30

Portions of the Sample 1 and Comparative Sample A polymers werecompounded on a 2-roll rubber mill in the proportions shown in TableIII. Curing characteristics and tensile properties of the cured polymersare also shown in Table III. The Sample 1 polymer, prepared by thesuspension polymerization process of this invention with controlledincorporation of a cure site monomer, exhibits a much higher cure state(as evidenced by M_(H)) and improved physical properties compared toComparative Sample A which was prepared according to the suspensionpolymerization process of the prior art.

TABLE III Sample 1 Comp. Sample A Formulation, phr¹: Polymer 100 100 MTBlack, Thermax FF N990 30 30 Ca(OH)₂, Rhenofit CF 5 5 Triallylisocyanurate 2.7 2.7 Peroxide, Luperox 101XL 45 3.75 3.75 Process aid,Armeen 18D 0.5 0.5 Process aid, VPA 2² 1 1 Cure Characteristics (ODR):M_(L), dN · m 6.9 3.4 M_(H), dN · m 59.7 45.7 T_(S)2, minutes 0.72 0.83T50, minutes 1.24 1.25 T90, minutes 2.84 2.72 Tensile Properties T_(B),MPa 17.3 14.9 E_(B), % 224 431 M₁₀₀, MPa 5.5 2.7 Hardness, Shore A 74 70Compression set, % 70 h/200° C. 25 47 ¹“phr” denotes parts by weight perhundred parts rubber ²Rice bran wax (available from DuPont DowElastomers L.L.C.)

Example 2

Sample 2 was prepared by the process of this invention in the followingmanner. The 40-liter reactor of Example 1 was charged with 20 liters ofwater containing 14 g methyl cellulose (M_(n) about 17,000) and washeated to 50° C. Gaseous monomers were charged as listed below to bringthe reactor pressure to 2.56 MPa:

Monomer Amount, g Wt. % TFE  183  6.3 VF₂  872 29.8 HFP 1870 63.9 Total2925

The polymerization was initiated by adding a solution of 20 g IPP in 80g methyl acetate. A solution of 36 g methylene iodide in 44 g methylacetate was also charged to the reactor. Approximately one third of themethylene iodide solution was added at initiation and the balance wasadded during the feed of the first 1800 g of incremental monomer.

A gaseous incremental major monomer mixture was fed in a manner thatmaintained constant reactor pressure at the controlled temperature of50° C. Liquid cure site monomer BTFB was fed in a controlled ratio tothe incremental gaseous monomer feed, according to the procedure ofExample 1. BTFB was initially fed at a ratio of 0.35 wt. % ofincremental monomer. The ratio was gradually increased to 0.75 wt. %.This resulted in an overall average ratio of 0.60 wt. %, based on totalincremental monomer fed. Polymerization rate was approximately equal toincremental feed rate and increased from approximately 100 g/h initiallyto approximately 1000 g/h after 10 hours. A total of 14,278 gincremental monomer was fed over a 20-hour reaction period in theamounts shown:

Monomer Amount, g Wt. % TFE 2736 19.2 VF₂ 7056 49.4 HFP 4486 31.4 Total14278 

The polymerization was terminated after 20 hours by discontinuing theincremental monomer feed. The resulting polymer slurry was filtered andwashed. Total polymer recovery was 15,435 g. Major monomer compositionwas determined by FTIR. Concentration of iodine and bromine cure siteswas determined by x-ray fluorescence. Polymer composition was 22.1 wt. %TFE, 51.4 wt. % VF2, 25.7 wt. % HFP, 0.54 wt. % BTFB, and 0.20 wt. % I.These values were close to the goal composition set by incrementalmonomer feeds. Polymer inherent viscosity was 0.73, ML-10 (121° C.) was42, and T_(g) was −19° C.

As in Example 1, addition of cure site monomer BTFB in a closelycontrolled ratio to the incremental gaseous monomer feed allowed thepolymerization to proceed at a satisfactory rate to form a highmolecular weight polymer having a homogeneous distribution of cure sitesfor good curing characteristics.

Example 3

The 40-liter reactor of Example 1 was charged with 20 liters of watercontaining 14 g methyl cellulose (M_(n) about 17,000). The reactorcontents were heated to 50° C. Gaseous monomers were charged in theamounts listed below to bring the reactor pressure to 2.14 MPa:

Monomer Amount, g Wt. % TFE 125  5.0 VF₂ 625 25.0 PMVE 1000  40.0 2-HPFP750 30.0 Total 2500 

The polymerization was initiated by adding a solution of 40 g IPP in 160g methyl acetate.

A gaseous incremental monomer mixture was fed in a manner so as tomaintain constant reactor pressure at the controlled temperature of 50°C. The gaseous cure site monomer, 2H-pentafluoropropylene (2-HPFP), wasfed along with the major gaseous monomers. Incremental feed rateincreased from about 500 g/h initially to about 2900 g/h after 4 hours,corresponding to the maximum output of the compressor. Polymerizationrate increased to approximately 3460 g/h at the end of the reactionperiod of 6 hours. Reactor pressure decreased to 1.36 MPa as a portionof the initial monomer charge reacted. A total of 12,520 g incrementalmonomer was fed over a 6-hour reaction period in amounts shown below:

Monomer Amount, g Wt. % TFE 1252 10.0 VF₂ 6751 54.0 PMVE 4257 34.02H-PFP  250  2.0 Total 12,520  

The polymerization was terminated after 6 hours by discontinuing theincremental monomer feed. The resulting polymer slurry was filtered andwashed. Total polymer (Sample 3) recovery was 14,280 g. Polymercomposition and properties are shown in Table IV. The ratio of 2-HPFP toPMVE was determined by ¹⁹F NMR.

TABLE IV Inherent viscosity 0.95 Mooney viscosity, ML (1 + 10)(121° C.)71 TFE, wt. % 11.4 VF₂, wt. % 52.6 PMVE, wt. % 33.7 2-HPFP, wt. % 2.3Tg, ° C. −29

A VF₂/TFE/PMVE/2-HPFP copolymer (Comparative Sample B) was prepared byemulsion polymerization in a 2-liter continuous stirred tank reactor at110° C., 6.2 MPa with a residence time of 40 minutes. Several aqueoussolutions were fed to the reactor at a rate of 3.00 liters/hour so as tointroduce into the reactor 1.27 g/hour ammonium persulfate initiator,0.91 g/hour NaOH, 1.70 g/hour ammonium perfluorooctanoate surfactant,and 0.35 g/hour isopropyl alcohol transfer agent. Monomers were fed tothe reactor at a rate of 836 g/hour, the feed stream being composed of7.0 wt. % TFE, 53.1 wt. % VF₂, 36.8 wt. % PMVE, and 3.1 wt. % 2-HPFP.Polymerization rate was 812 g/hour at 97% conversion producing anemulsion containing 21.4 wt. % solids.

Polymer was isolated by coagulation with potassium aluminum sulfatesolution, followed by washing and drying of the crumb. Polymercomposition was 7.2 wt. % TFE, 54.1 wt. % VF₂, 36.8 wt. % PMVE, and 1.9wt. % 2-HPFP. Mooney viscosity, ML-10 (121° C.), was 98.

The Sample 3 and Comparative Sample B polymers were compounded on a2-roll mill with curative, filler and process aid in the proportionsshown in Table V. Curing characteristics and tensile properties of thecured polymers are also shown in Table V. Physical properties of thecured polymers of this example were measured on samples which had beenpress cured for 4 minutes at 180° C. and then post cured in an air ovenfor 24 hours at 230° C.

Sample 3 (suspension polymer) having no ionic endgroups cured faster andgave lower compression set than the emulsion polymer (Comparative SampleB), having ionic endgroups.

TABLE V Comp. Sample B Sample 3 Formulation, phr: Polymer 100 100 Tremin283600 EST filler³ 45 45 MT Black, Thermax FF N990 — 2.5 TiO₂ Ti-PureR960 2 — Cromophtal Blue 4GNP⁴ 2.5 — Calcium Oxide VG 6.0 6.0 MgO,Elastomag 170 1.0 1.0 Molecular Sieve 13X 3.0 3.0 Bisphenol AF 2.0 2.0Tetrabutyl ammonium hydrogen 0.4 0.5 sulfate Process aid, VPA 2² 1.0 1.0Cure Characteristics (MDR): M_(L), dN · m 3.6 3.0 M_(H), dN · m 25.227.7 T_(S)2, minutes 0.53 0.33 T90, minutes 2.24 1.51 Tensile PropertiesT_(B), MPa 12.2 12.2 E_(B), % 141 156 M₁₀₀, MPa 9.4 8.7 Hardness, ShoreA 74 74 Compression set, % 70 h/200° C. 42 37 ³Epoxysilane treatedwollastonite mineral filler ⁴Blue pigment

Example 4

The 40-liter reactor of Example 1 was charged with 20 liters of watercontaining 14 g methyl cellulose (M_(n) about 17,000) and the contentswere heated to 50° C. Gaseous monomers were charged in the amounts shownbelow to bring the reactor pressure to 1.55 MPa:

Monomer Amount, g Wt. % TFE  45  3.0 VF₂ 405 27.0 PMVE 600 40.0 2-HPFP450 30.0 Total 1500 

The polymerization was initiated by adding a solution of 40 g IPP in 160g methyl acetate.

A gaseous incremental monomer mixture was fed to the reactor in a mannerso as to maintain constant reactor pressure at the controlledtemperature of 50° C. The gaseous cure site monomer,2H-pentafluoropropylene (2-HPFP), was fed along with the major gaseousmonomers. Incremental feed rate, approximately equal to polymerizationrate, increased from about 176 g/hour initially to about 1956 g/hour atthe termination of the polymerization period of 10.7 hours. A total of12,000 g incremental monomer was fed over the 10.7-hour reaction periodin the amounts shown below:

Monomer Amount, g Wt. % TFE  480  4.0 VF₂ 6960 58.0 PMVE 4320 36.02-HPFP  240  2.0 Total 12,000  

The polymerization was terminated after 10.7 hours by discontinuing theincremental monomer feed. The resulting polymer slurry was filtered andwashed. Total polymer (Sample 4) recovery was 12.0 kg. Polymercomposition and properties are shown in Table VI. The ratio of 2-HPFP toPMVE was determined by ¹⁹F NMR.

TABLE VI Inherent viscosity 0.81 Mooney viscosity, ML (1 + 10)(121° C.)43 TFE, wt. % 3 VF₂, wt. % 59 PMVE, wt. % 36 2-HPFP, wt. % 2 Tg, ° C.−31

The curing characteristics and physical properties of cured compounds(mixed on a 2-roll rubber mill) are shown in Table VII for thebisphenol-curable polymer of Sample 4 and Comparative Sample C, Viton®GLT fluoroelastomer (commercially available from DuPont Dow ElastomersL.L.C.), a peroxide-curable fluoroelastomer prepared by continuousemulsion polymerization. The polymer of Comparative Sample C has anapproximate composition 10 wt. % TFE, 54 wt. % VF₂, 35 wt. % PMVE, and1.2 wt. % BTFB, and has an ML-10 (121° C.) of about 90. Physicalproperties of the cured compositions were measured on samples which hadbeen press cured for 4 minutes at 180° C. and then post cured in an airoven for 24 hours at 230° C.

Cure rate and physical properties of the bisphenol-curable suspensionpolymer (Sample 4) were similar to those of Comparative Sample C, thecommercial peroxide-curable polymer GLT. However, the bisphenol-curablecompound gave much improved mold release and better retention ofproperties after heat aging at 250° C.

TABLE VII Comp. Sample C Sample 4 Formulation, phr: Polymer 100 100Tremin² 283600 EST filler³ 45 45 MT Black, Thermax FF N990 2.5 2.5Calcium Oxide VG — 6.0 MgO, Elastomag 170 — 1.0 Molecular Sieve 13X —3.0 Bisphenol AF — 2.0 Tetrabutyl ammonium hydrogen — 0.5 sulfateCa(OH)₂, Rhenofit CF 5 — Peroxide, Luperox 101XL 45 2 — Triallylisocyanurate, Diak 7 4 — Process aid, Armeen 18D 0.5 — Process aid, VPA2 1.0 1.0 Cure Characteristics (MDR): M_(L), dN · m 3.9 2.2 M_(H), dN ·m 22.9 23.8 T_(S)2, minutes 0.52 0.29 T50, minutes 0.93 0.42 T90,minutes 2.74 2.70 Tensile Properties T_(B), MPa 18.5 12.0 E_(B), % 153176 M₁₀₀, MPa 14.3 8.2 Hardness, Shore A 75 74 Compression set, % 70h/200° C. 32 37

What is claimed is:
 1. A suspension polymerization process for producinga fluoroelastomer having a selected molar ratio of copolymerized monomerunits, said fluoroelastomer comprising copolymerized units of vinylidenefluoride major monomer, at least one other copolymerizable fluorinatedmajor monomer, and at least one cure site monomer, comprising the stepsof: (A) charging a reactor with a quantity of an aqueous mediumcomprising a suspension stabilizer, said suspension stabilizer beingpresent in said aqueous medium at a concentration of 0.001 to 3 parts byweight per 100 parts of said aqueous medium; said quantity of aqueousmedium being such that a sufficient vapor space is left in said reactorfor receiving gaseous monomer; (B) charging the vapor space in saidreactor with an initial quantity of a gaseous monomer mixture comprisingvinylidene fluoride major monomer and at least one other fluorinatedmajor monomer; and continuously mixing said aqueous medium and saidmonomer mixture to form a dispersion; (C) initiating polymerization ofsaid monomers at a temperature of 45° C. to 70° C. by adding to saiddispersion an oil soluble organic peroxide polymerization initiator inan amount of 0.001 to 5 parts by weight per 100 parts of said aqueousmedium, said initiator being added as a solution consisting essentiallyof 0.1 to 75 wt. % of an oil soluble organic peroxide in a water-solublesolvent, said solvent selected from the group consisting of compounds ofthe formulas R₁OH, R₂COOR₁, and R₁COR₃, where R₁ and R₃ are methyl ort-butyl groups, and R₂ is hydrogen, a methyl group or a t-butyl group;and (D) incrementally feeding to said reactor, during polymerization, soas to maintain a constant pressure in said reactor, said major monomersand at least one cure site monomer, said major monomers and said curesite monomer being fed to the reactor in said selected molar ratio untila fluoroelastomer product having a number average molecular weight ofbetween 50,000 to 2,000,000 daltons is obtained.
 2. A process of claim 1wherein at least one of said other copolymerizable fluorinated majormonomers is selected from the group consisting of hexafluoropropylene,tetrafluoroethylene, chlorotrifluoroethylene and a perfluoro(alkylvinyl) ether.
 3. A process of claim 1 wherein said cure site monomer isselected from the group consisting of 2-hydropentafluoropropylene, anon-conjugated diene, a bromine-containing olefin, an iodine-containingolefin, a bromine-containing unsaturated ether, and an iodine-containingunsaturated ether.
 4. A process of claim 1 wherein said fluoroclastomerproduct comprises copolymerized units of 30-65 wt. % vinylidenefluoride, 30-40 wt. % perfluoro(methyl vinyl) ether, 3-30 wt. %tetrafluoroethylene and 0.5-3 wt. % of a cure site monomer selected fromthe group consisting of 2-hydropentafluoropropylene;4-bromo-3,3,4,4-tetrafluorobutene-1; 4-iodo-3,3,4,4-tetrafluorobutene-1;and allyl iodide.
 5. A process of claim 1 wherein said fluoroelastomerproduct comprises copolymerized units of 30-65 wt. % vinylidenefluoride, 25-40 wt. % hexafluoropropylene, 3-30 wt. %tetrafluoroethylene and 0.5-3 wt. % of a cure site monomer selected fromthe group consisting of 4-bromo-3,3,4,4-tetrafluorobutene-1;4-iodo-3,3,4,4-tetrafluorobutene-1; and allyl iodide.
 6. A process ofclaim 1 further comprising the step of charging the reactor at leastonce with a quantity of a chain transfer agent.
 7. A process of claim 6wherein said chain transfer agent is selected from the group consistingof methylene iodide; 1,4-diiodoperfluoro-n-butane; 1,6-diiodo-3,3,4,4,tetrafluorohexane; 1,3-diiodoperfluoropropane; 1,4-diiodoperfluorobutane; 1,6-diiodoperfluorohexane; 1,3-diiodo-2-chloroperfluoropropane;1,2-di(iododifluoromethyl)-perfluorocyclobutane;monoiodoperfluoroethane; monoiodoperfluorobutane;2-iodo-1-hydroperfluoroethane ; 1-bromo-2-iodoperfluoroethane;1-bromo-3-iodoperfluoropropane; and 1-iodo-2-bromo-1,1-difluoroethane.